1. Field of the Invention
The present invention relates to a method for driving an optical pickup apparatus that irradiates an optical disc with laser light to record or playback data.
2. Description of the Related Art
As disc-shaped recording media, many types of optical disc such as a compact disc (CD), a digital versatile disc (DVD), a high-density DVD (HD-DVD) and a Blu-ray Disc have been put into practical use in the past. With these types of optical discs, the wavelength of laser light used, the depth of the transparent layers extending down to the recording layer, and the like differ, so a single or a plurality of objective lenses are used to handle a plurality of types of optical disc.
Furthermore, an optical pickup apparatus is used which irradiates an optical disc with laser light to record or playback data. The optical pickup apparatus is equipped with an objective lens that focuses laser light at a spot on the signal recording layer of the optical disc.
Therefore, focus servo control to focus the light beam from the light source through the objective lens onto the recording layer of the optical disc and tracking servo control to track and scan at the fine track pitch of the optical disc are performed.
Moreover, in order to align the focus of the light beam on a specified recording layer, the focus error signals (FE signals) obtained in the light reflected from the optical disc are observed while moving the objective lens in the focus direction using an actuator to move the focus of the light beam emitted from the light source and emanating from the objective lens in the focus direction, thereby performing a focus search to detect the position at which the focus of the light beam coincides with the recording layer of the optical disc.
In addition, during a focus servo control operation to align the focus of the light beam on a different recording layer, S-shaped waveforms appearing in the FE signals are detected at the time that the focus of the light beam passes through the surface and recording layer, respectively, of the optical disc. Furthermore, the position at which the focus of the light beam coincides with the recording layer is detected based on the S-shaped waveform corresponding to the recording layer of the optical disc. Moreover, the substrate thickness, the protective layer thickness, and the number of recording layers of the optical disc are detected based on the detection time differences among the various S-shaped waveforms corresponding to the surface and recording layers of the optical disc.
Furthermore, there are also known spherical aberration compensation mechanisms which perform compensation of spherical aberrations by moving along the direction of the optical axis a luminous flux-changing movable lens (collimating lens, expander lens, or the like) that changes the convergence and scattering state of the luminous flux incident upon the object lens, thus adjusting the convergence and scattering state of the laser light passing through these lenses. Moreover, in a multilayer optical disc in which a plurality of recording layers are laminated, spherical aberration is generated due to the distance between recording layers, such that optimal spherical aberration compensation is generally performed for each surface when performing servo control for each recording layer.
To this end, after moving a collimating lens from an original position to a specified position and performing the spherical aberration compensation corresponding to the specified recording layer, the objective lens is moved in the focusing direction while counting the number of S-shaped waveforms appearing in the FE signal to perform the focus servo control for the target recording layer.
In order to move the collimating lens from the original position to the specified position, the collimating lens is moved to the specified position by driving a stepper motor a specified number of pulses after confirming that the collimating lens is in the original position, or after temporarily returning the collimating lens to the original position, for instance.
In addition, an optical pickup apparatus has been proposed which is designed to detect a reference position of the collimating lens by providing an optical detector that receives a portion of the light beam passing through the collimating lens and utilizing changes in the detection situation of this optical detector (see, for example, Japanese Patent Application Laid-Open Publication No. 2008-293601).
In the case of temporarily returning the collimating lens to the original position and then moving the collimating lens to the specified position, in order to make restoration to the original position possible regardless of the position of the collimating lens, the stepper motor that can be driven back and forth between the original position and the drive-limit position is driven by the number of pulses required to return the collimating lens from the drive-limit position to the original position.
To do this, a nut made of plastic is installed so as to be movable by being threaded onto the screw shaft that is rotary-driven by the stepper motor and which allows gear-tooth skipping and disengaged rotation in a state of contact with a specified stopping member, and the nut is moved along the screw shaft to move the collimating lens.
Furthermore, in order to prevent the phenomenon of locking of the nut when it exceeds its movement limit, an optical disc device has been proposed in which the nut is split to adopt a structure with its diameter being capable of enlarging or shrinking, and a tapered member is caused to contact with the nut at the end portion of movement, thereby enlarging the nut diameter (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-104668).